1. Field of the Invention
The present invention relates to a method for preparing finely divided solid polymer beads that are insoluble in water, in organic solvents, and in mixtures of water and organic solvents. The invention also relates to the use of such polymer beads as spacing agents in a sheet or web material such as a recording material e.g. a light-sensitive silver halide material.
2. Description of the Prior art
It is known that hydrophobic resin sheet and web materials having a low conductivity readily become charged electrostatically, especially in a relatively dry atmospheric environment, as a result of friction against dielectric materials and/or of contact with electrostatically chargeable transport means e.g. rollers.
Sheets and webs of hydrophobic resins e.g. polyesters or cellulose triacetate are commonly used as support element of recording materials e.g. a light-sensitive silver halide material. Such materials are subjected to frictional contact with other elements during their manufacture e.g. during a coating or sizing stage, and during their use e.g. during the recording of information in a flow camera or movie camera and during image processing and image projection as can be the case for photographic silver halide recording materials.
Especially in the reeling up or unreeling of dry photographic film in a camera or projector high friction may occur, which may give rise to electrostatic charges that may attract dust or cause sparking. In unprocessed photographic silver halide recording materials sparking causes developable fog and thus degrades the image quality.
For the purpose of reducing electrostatic charging of sheet or web materials comprising a hydrophobic resin support carrying at least one silver halide emulsion layer and at the same time not impairing their transparency, ionic compounds have been incorporated into these materials e.g. in the silver halide emulsion layer(s). However, ionic compounds migrate out of a silver halide emulsion layer during its different wet processing treatments. To avoid such migration preference has been given to antistatic high molecular weight polymeric compounds having ionic groups at frequent intervals in the polymer chain (cfr "Photographic Emulsion Chemistry" by G.F. Duffin, The Focal Press - London and New York (1966) - Focal Press Ltd., page 168).
From U.S. patent application no. 3,525,621 it is known that antistatic properties can be given to an aqueous coating composition by incorporating therein almost any silica sol, but preferably silica having a high surface area of the order of magnitude of 200-235 m2 per gram in combination with an alkali metal salt of an alkylaryl polyethersulphonate. The applied alkali metal salt of an alkylaryl polyethersulphonate is a highly water-soluble compound that leaches out during photographic aqueous processing so that the antistatic character of the support, on which the antistatic composition is coated, decreases markedly. As a consequence, the antistatic behaviour of the processed and dried material is unsatisfactory, since the residual colloidal silica fails to sufficiently reduce the surface resistivity.
According to EP-A 334,400 a sheet or web material is provided, which comprises an optionally subbed hydrophobic resin or paper support coated with at least one hydrophobic resin layer and having on at least one side thereof an outermost antistatic layer containing colloidal silica, wherein the antistatic layer is free from any cationic surfactant and consists for at least 70 % by weight of colloidal silica having an average particle size not higher than 10 nm and a surface area of at least 300 m2 per gram, the colloidal silica being present at a coverage of at least 50 mg per m2. When such sheet or web material is part of a photographic silver halide material, the antistatic layer retains a low surface resistivity after wet photographic processing and drying but has the disadvantage of sticking under mild pressure to hydrophilic colloid layers such as layers containing gelatin. This is particularly disturbing when the antistatic layer of the photographic material during storage or reeling up thereof comes into contact with other layers containing gelatin. For instance, when the photographic material is in rolled up state, the antistatic layer, which is at the rear side of the photographic silver halide material, can stick to the emulsion layer(s) on the opposite side. During unrolling of the photographic material damage can be caused since parts of the emulsion layer(s) or of the antistatic layer can be torn off.
According to the unpublished EP application 90200482.9 entitled "Sheet or web material having antistatic properties" and filed on Mar. 1, 1990 the sticking of an above-mentioned antistatic layer containing colloidal silica to layers containing gelatin is avoided by covering the antistatic layer with a layer that has a poorer sticking power to a gelatin layer than said antistatic layer containing colloidal silica, said covering layer containing at least 50% by weight of a water-insoluble synthetic polymer, e.g. polymethyl methacrylate, coated at a ratio in dried state of 0.01 to 2 g/m2. Said covering layer is coated from a coating solution, the solvent medium of which consists of at least one organic solvent.
However, such a covering polymer layer is smooth. In various processes such as for the coating of a composition on a surface of a sheet or web material, which on its rear surface carries such a smooth covering layer, the web material when travelling lengthwise at high speed tends to float when nearing a rotating roller. Such floating is caused by the fact that air is picked up by the moving surfaces of the web and the roller and that some of this air becomes entrapped in the wedge-shaped space where the web approaches the roller surface. Owing to this floating an irregular application of coating composition is induced.
Moreover, because of the smoothness of the covering layer the sheet material tends to block while in contact with other members or surfaces.
Blocking could be avoided by incorporating in the covering layer of the web material polymer beads that protrude from said layer and thus act as spacing agents that provide a safe distance between said layer and contacting objects and thus guarantee an easy conveyability of the web material in respect of contacting surfaces. Such beads have been described e.g. in U.S. patent application no. 4,614,708. However, known polymer beads are not resistant against organic solvents, so that coating of a composition comprising such beads in a solution of waterinsoluble synthetic polymer in organic solvent(s) is not possible. The beads would dissolve completely or at least start sticking to each other and form large conglomerations.